max-lines-per-function

Enforce a maximum number of lines of code in a function

Some people consider large functions a code smell. Large functions tend to do a lot of things and can make it hard following what’s going on. Many coding style guides dictate a limit of the number of lines that a function can comprise of. This rule can help enforce that style.

Rule Details

This rule enforces a maximum number of lines per function, in order to aid in maintainability and reduce complexity.

Why not use max-statements or other complexity measurement rules instead?

Nested long method chains like the below example are often broken onto separate lines for readability:

function() {
    return m("div", [
        m("table", {className: "table table-striped latest-data"}, [
            m("tbody",
                data.map(function(db) {
                    return m("tr", {key: db.dbname}, [
                        m("td", {className: "dbname"}, db.dbname),
                        m("td", {className: "query-count"},  [
                            m("span", {className: db.lastSample.countClassName}, db.lastSample.nbQueries)
                        ])
                    ])
                })
            )
        ])
    ])
}

• max-statements will only report this as 1 statement, despite being 16 lines of code.
• complexity will only report a complexity of 1
• max-nested-callbacks will only report 1
• max-depth will report a depth of 0

Options

This rule has the following options that can be specified using an object:

• "max" (default 50) enforces a maximum number of lines in a function.

• "skipBlankLines" (default false) ignore lines made up purely of whitespace.

• "skipComments" (default false) ignore lines containing just comments.

• "IIFEs" (default false) include any code included in IIFEs.

Alternatively, you may specify a single integer for the max option:

"max-lines-per-function": ["error", 20]

is equivalent to

"max-lines-per-function": ["error", { "max": 20 }]

code

Examples of incorrect code for this rule with a max value of 2:

/*eslint max-lines-per-function: ["error", 2]*/
function foo() {
    var x = 0;
}

/*eslint max-lines-per-function: ["error", 2]*/
function foo() {
    // a comment
    var x = 0;
}

/*eslint max-lines-per-function: ["error", 2]*/
function foo() {
    // a comment followed by a blank line

    var x = 0;
}

Examples of correct code for this rule with a max value of 3:

/*eslint max-lines-per-function: ["error", 3]*/
function foo() {
    var x = 0;
}

/*eslint max-lines-per-function: ["error", 3]*/
function foo() {
    // a comment
    var x = 0;
}

/*eslint max-lines-per-function: ["error", 3]*/
function foo() {
    // a comment followed by a blank line

    var x = 0;
}

skipBlankLines

Examples of incorrect code for this rule with the { "skipBlankLines": true } option:

/*eslint max-lines-per-function: ["error", {"max": 2, "skipBlankLines": true}]*/
function foo() {

    var x = 0;
}

Examples of correct code for this rule with the { "skipBlankLines": true } option:

/*eslint max-lines-per-function: ["error", {"max": 3, "skipBlankLines": true}]*/
function foo() {

    var x = 0;
}

skipComments

Examples of incorrect code for this rule with the { "skipComments": true } option:

/*eslint max-lines-per-function: ["error", {"max": 2, "skipComments": true}]*/
function foo() {
    // a comment
    var x = 0;
}

Examples of correct code for this rule with the { "skipComments": true } option:

/*eslint max-lines-per-function: ["error", {"max": 3, "skipComments": true}]*/
function foo() {
    // a comment
    var x = 0;
}

IIFEs

Examples of incorrect code for this rule with the { "IIFEs": true } option:

/*eslint max-lines-per-function: ["error", {"max": 2, "IIFEs": true}]*/
(function(){
    var x = 0;
}());

(() => {
    var x = 0;
})();

Examples of correct code for this rule with the { "IIFEs": true } option:

/*eslint max-lines-per-function: ["error", {"max": 3, "IIFEs": true}]*/
(function(){
    var x = 0;
}());

(() => {
    var x = 0;
})();

When Not To Use It

You can turn this rule off if you are not concerned with the number of lines in your functions.

Related Rules

• complexity
• max-depth
• max-lines
• max-nested-callbacks
• max-params
• max-statements
• max-statements-per-line

Version

This rule was introduced in ESLint v5.0.0.

Resources

• Rule source
• Tests source